The invention of ultrafast lasers in 1965 led to the desire for new techniques to measure the duration of ultrashort optical pulses. Direct measurement techniques using photodetectors and oscilloscopes are inadequate to temporally resolve the pulses being produced by ultrafast lasers, typically less than 1 ns in duration. An indirect technique with subpicosecond time resolution has been proposed and demonstrated, however. This technique is based on the nonlinear process of second-harmonic generation (SHG). The optical pulse is divided into two beams, which travel different paths before being recombined in a nonlinear crystal, to generate a new second harmonic pulse that is then detected. The second harmonic pulse represents the autocorrelation of the ultrafast pulse at a particular temporal offset. One of the path lengths of the two beams is varied so that the second harmonic pulses may sketch out the autocorrelation function of the ultrafast pulses. The autocorrelation technique for pulse measurement does not provide a way of measuring pulse shape but instead gives correlation functions, which can be used to resolve pulse duration as shown in Equation 1.
                                          I            auto                    ⁡                      (            τ            )                          ∝                                                                        ∫                                  -                  ∞                                ∞                            ⁢                              E                ⁢                                                                  ⁢                                                      (                                          t                      -                      τ                                        )                                    ·                  E                                ⁢                                                                  ⁢                                  (                  t                  )                                ⁢                                                                  ⁢                                  ⅆ                  t                                                                          2                                    (        1        )            
Unfortunately, Equation 1 illustrates that the autocorrelation function is always temporally symmetric, even if the laser pulse E(t) has an asymmetric shape. Therefore this approach is not desirable for determining the shape of ultrafast laser pulses.
A Frequency-Resolved-Optical-Gating (FROG) provides a way to solve this problem. An autocorrelation technique as previously described only captures the intensity information of a laser pulse. To fully characterize the pulse, it is desirable to capture phase information as well as intensity information. To solve this problem, a FROG device was proposed to measure the pulse shape. A FROG is also an autocorrelator, but what differentiates a FROG from a typical autocorrelator is that it captures the spectrum of the signal, as opposed to just the intensity for each delay position. The phase information of the pulse is contained in the spectrum. Thus, the pulse shape can be retrieved from the FROG trace.
When measuring a laser pulse shape use this FROG method, it is desirable to have good spectral resolution to ensure that the necessary phase information recorded. A typical femtosecond laser pulse has a wavelength bandwidth of less than 1 nm. Such a bandwidth may be easily resolved with presently available spectrometers, and the FROG technique works well for determining the pulse shape and duration of femtosecond laser pulses. However, FROG techniques do not work as well for picosecond laser pulses. For example, a transform-limited 20 ps laser pulse at 1053 nm has a bandwidth of 0.1 nm, which may be difficult to resolve accurately using a spectrometer.
Another approach that has been proposed to measure the pulse shape of ultrafast pulses is to image the pulses using a streak camera. This method works well for longer pulses, but may have difficulties with obtaining highly precise and accurate pulse shape measurements for pulse widths less that about 1 ns due to jitter in the timing circuitry of the camera. Improvements in these circuits may extend the range of this method, but some jitter is likely to remain.
The present invention provides a method that may be used to accurately measure pulse shapes for optical pulses over a broad range of pulse widths, including picosecond laser pulses that are difficult to measure by other techniques.